Non-inferiority Study of Frexalimab Subcutaneous Administration Compared to Intravenous Administration in Adult Participants With Multiple Sclerosis

This is a randomized, open-label, parallel, Phase 3 study with 2-arms for treatment. The purpose of this study is to evaluate SC administration of frexalimab every 4 weeks (q4w) compared to IV administration of frexalimab q4w in male and female participants with RMS and nrSPMS (aged 18 to 60 years at the time of enrollment). People diagnosed with MS are eligible for enrollment as long as they meet all the inclusion criteria and none of the exclusion criteria. Study details include: The study intervention duration will be 48 weeks (12 months) for Parts A and B combined. Optional Part C will last until the initiation of a long term safety study for Frexalimab.The follow up duration after the end of study intervention (in case of discontinuation) will be 6 months. The number of scheduled visits (Parts A and B) will be 17 or 11 for participants receiving frexalimab SC or IV, respectively, with an on-site visit frequency of every month between Week 4 and Week 24 in Part A, then every 1 to 3 months in Part B, then every 6 months in Part C. Participants discontinuing treatment before the End of Study will have an additional 3 follow-up visits.

Inclusion Criteria: The participant must qualify for inclusion per either Group A or B criteria as detailed below, meeting all the inclusion criteria of the applicable group: Group A (RMS) * The participant must be 18 to 55 years of age, inclusive, at the time of signing the informed consent. * The participant must have been diagnosed with RMS in accordance with the 2017 revised McDonald criteria. * The participant must have an Expanded Disability Status Scale (EDSS) score of ≤5.5 at the first visit (Screening Visit). * The participant must have at least 1 of the following prior to screening: * 1 documented relapse within the previous year OR * 2 documented relapses within the previous 2 years, OR * 1 documented Gd enhancing lesion on an MRI scan within the previous year. Group B (nrSPMS) * Participant must have a previous diagnosis of RRMS in accordance with the 2017 revised McDonald criteria * The participant must be 18 to 60 years of age, inclusive, at the time of signing the informed consent. * The participant must have a current diagnosis of SPMS in accordance with the clinical course criteria revised in 2013. * The participant must have documented evidence of disability progression observed during the 12 months before screening. * The participant must have an absence of clinical relapses for at least 24 months. * The participant must have an EDSS score between 3.0 and 6.5 points, inclusive, at the first visit (Screening Visit). Participants from Group A and Group B are eligible to be included in the study only if all of the following criteria also apply: \- Contraceptive use by men and women should be consistent with local regulations regarding the methods of contraception for those participating in clinical studies. Exclusion Criteria: * The participant has been diagnosed with primary progressive MS according to the 2017 revision of the McDonald diagnostic criteria. * The participant has a history of infection or may be at risk for infection: * Fever within 28 days of the Screening Visit * Presence of psychiatric disturbance or substance abuse * History, clinical evidence, suspicion or significant risk for thromboembolic events, as well as myocardial infarction, stroke, and/or antiphospholipid syndrome and any participants requiring antithrombotic treatment. * Current hypogammaglobulinemia defined by Ig levels (IgG and/or IgM) below the LLN at screening or a history of primary hypogammaglobulinemia. * A history or presence of disease that can mimic MS symptoms. * The participant has a contraindication for MRI. The above information is not intended to contain all considerations relevant to a patient's potential participation in a clinical trial.